The venerable Western Hemlock - Washington’s state tree - has been in decline in the Puget Lowlands since 2015. The trees are dying - not everywhere at at once, but intermittently, often in clusters, throughout low elevations west of the Cascades.
Researchers initially attributed the mortality to Rhizoctonia butini, a fungus in the order Cantharellales. This hypothesis has since fallen out of favor (Jared LeBoldus, OSU Corvallis, personal communication). No subsequent explanation has emerged.
We frequently hear this mortality attributed to normal old age senescence, or climate change.
We are not so sure. Casual observation in Seward Park’s 120-acre old-growth urban forest suggested that age and climate did not explain hemlock mortality, in that we see areas with many dead trees, and we see many more areas dense with healthy trees. A mix of tree ages is found in both kinds of sites. We can assume that climate and weather patterns are constant across this small forest.
If the observed contrast holds up, and if the population of trees in the two contrasting areas are otherwise identical - by age, density, and site characteristics - then we have an ideal site for studying the regional decline. Seward provides a natural experiment.
Between January and March, two Univeristy of Washington students from the Program on the Environment in their senior Capstone Research Project, along with a Garfield High School junior spent about nine weeks in the forest, collecting data to test our hypothesis.
We also used data collected in the summer of 2021 by CHOOSE 180 interns.
Our citizen and student science sets the stage for in-depth research. These are the steps we have taken:
We started, as mentioned above, with all the trees in two dense hemlock sites. Preliminary statistics confirmed our intuition, as these boxplots show.
Subsequent mapping and analysis refined these results - see below.
We soon realized the importance of broadening our study. Here is a screenshot of our efforts at an intermediate stage, the blue circles marking the garden and graveyard, with many additional trees added.
In subsequent work, shown below, the blue, visually estimated boundaries are replaced by analytical boundaries calculated by kernel density methods.
“Overplotting” - the display of abundant overlapping points - makes it hard (quite literally), to see the forest for the trees. We see this in the map above, and in the final interactive version available here. Kernel density mapping solves the problem: large patterns to emerge. These three panels below do this, showing the concentration of all (left), of affected (middle) and of healthy (left) hemlocks across the forest.
These three density maps confirm our informal observation, and hypothesis, that contrasting health and Seward’s version of hemlock decline is not straightforwardly explained by senesence or weather patterns.
To strengthen our case, we defined two contiguous regions with
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We predict that foliar and mycorrhizal DNA sequencing, entomological studies, and root rot analyses may all be needed to understand this problem. Once understood, remedies may become possible.